Size reduction, or comminution of particulate materials is commonly practiced in the mining and mineral industries. For example, beneficiation of ores from a mine commonly require that the ore be subject to comminution in order to reduce the particle size of the ore and to expose the desired mineral faces for the beneficiation process. This is especially so in relation to flotation processes for producing concentrates from ores, for leaching of minerals from ores or concentrates, as well as physical separation processes such as gravity, electrostatic and magnetic separation. Similarly, a number of other mineral treatment processes require size reduction of an ore or concentrate in order to increase the kinetics of the mineral treatment process to economical rates.
Grinding is one frequently used method for size reduction or comminution of particulate materials. Grinding mills typically include a grinding chamber to which the particulate material is added. An outer shell of the grinding chamber may be rotated, or an internal mechanism in the grinding chamber may be rotated (or both). This causes stirring or agitation of the particulate material in the grinding chamber. A grinding medium may also be added to the grinding chamber. If the grinding medium is different to the particulate material being subjected to comminution, the grinding method is referred to exogenous grinding. If collisions between the particulate material itself causes the grinding action and no other grinding medium is added, it is known as autogenous grinding. A wide variety of grinding mills are known including bead mills, peg mills, ball mills, rod mills, colloid mills, fluid energy mills, cascade mills, stirred mills, agitated mills, SAG mills, AG mills, tower mills and vibrated mills.
U.S. Pat. Nos. 5,797,550 and 5,984,213 (the entire contents of which are incorporated herein by cross-reference) describe a grinding mill or an attrition mill which includes an internal classification zone in the grinding chamber. The mills described in these U.S. patents may be vertical shaft mills or horizontal shaft mills. A commercial embodiment of the mills described in these United States patents is sold under the trade name “IsaMill” by Xstrata Technology, a business division of the applicants in respect of the present application.
The feed material fed to a grinding mill and the product material removed from a grinding mill will have a particle size distribution. There are a number of ways of characterizing the particle size distribution of particulate material. For example, a graphical representation as to the cumulative mass percent passing a nominal size versus the particle size may be used. The nomenclature Dx is then used to denote the size at which weight percent, on a cumulative basis passes. For example, D80 refers to a particulate size distribution where 80% (on a cumulative basis) passes the nominated size. Thus, D80 equals 75 microns refers to a particulate size distribution in which 80% of the mass is finer than 75 microns.
IsaMill technology has been implemented to achieve ultrafine grinding of relatively fine feed particulate materials. The Isamill utilizes circular grinding discs that agitate the media and/or particles in a slurry. A classification and product separator keeps the grinding media inside the mill, allowing only the product to exit. Installations of IsaMills to date have used natural grinding media and directed to obtaining an ultrafine product having a D80 of below 19 microns, and most commonly a D80 of below 12 microns.
In grinding applications, the feed particulate material is typically referred to as F and the product particulate material is referred to as P. Thus, F50 refers to a feed sample where 50% passes the nominated size. Similarly, P98 equals 100 micrometers refers to a product size distribution where 98% of the mass is finer than 100 micrometers.
Size distribution curves in grinding applications, described as size versus cumulative percent passing on a log versus normal axis, are typically characterized by a single point on the curve, namely D80 (or 80% cumulative mass passing size). The P80 is a reasonable description of classical grinding and classification size distribution curves as the feed size distribution is progressively moved to the left on a log-linear scale as the particles are ground to finer sizes with traditional techniques.